This invention relates to a process for separating hydrogen from gas mixtures using a semi-permeable membrane derived from polycarbonate wherein the polycarbonate is derived in a significant portion from tetrahalobisphenols.
In various industries, it is necessary or highly desirable to separate one component from another in a gaseous stream. Processes used to perform such separations include cryogenics, pressure swing adsorption, chemical absorption, and membrane separations.
Membranes have been used to recover or isolate a variety of gases, including hydrogen, helium, oxygen, nitrogen, carbon monoxide, carbon dioxide, water vapor, hydrogen sulfide, ammonia, and light hydrocarbons. Applications of particular interest include the separation of hydrogen from gas mixtures such as mixtures containing nitrogen, carbon monoxide, carbon dioxide, and/or light hydrocarbons. For example, the separation and use of hydrogen is often necessary in various hydrocracker, hydrotreater, and catalytic cracking processes used in the oil refinery industry. Membranes can be used to achieve such separations.
Such membrane separations are based on the relative permeability of two or more gaseous components through the membrane. To separate a gas mixture into two portions, one richer and one leaner in at least one component, the mixture is brought into contact with one side of a semi-permeable membrane through which at least one of the gaseous components selectively permeates. A gaseous component which selectively permeates through the membrane passes through the membrane more rapidly than at least one other component of the mixture. The gas mixture is thereby separated into a stream which is enriched in the selectively permeating component or components and a stream which is depleted in the selectively permeating component or components. The stream which is depleted in the selectively permeating component or components is enriched in the relatively non-permeating component or components. A relatively non-permeating component permeates more slowly through the membrane than at least one other component of the mixture. An appropriate membrane material is chosen for the mixture so that some degree of separation of the gas mixture can be achieved.
Membranes for hydrogen separation have been fabricated from a wide variety of polymeric materials, including cellulose esters, polyimides, polyaramides, and polysulfones. An ideal gas separation membrane is characterized by the ability to operate under high temperature and/or pressure while possessing a high separation factor (selectivity) and high gas permeability. The problem is finding membrane materials which possess all the desired characteristics. Polymers possessing high separation factors generally have low gas permeabilities, while those polymers possessing high gas permeabilities generally have low separation factors. In the past, a choice between a high separation factor and a high gas permeability has been unavoidably necessary. Furthermore, some of the membrane materials previously used suffer from the disadvantage of poor performance under high operating temperatures and pressures. A membrane capable of separating hydrogen from light hydrocarbons which possesses high selectivity, high gas permeability, and ability to operate under extreme conditions of temperature and pressure is needed.